1. Field of the Invention
The present invention relates generally to laser technology for photolithography, and, more particularly, to optimization of extreme ultraviolet (EUV) light production.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (EUV) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nanometers (nm). EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (LPP), the required plasma can be produced by irradiating a target, such as a droplet, stream or cluster of material having the desired spectral line-emitting element, with a laser beam at an irradiation site.
The spectral line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid, This target is delivered to a desired irradiation site (e.g., a primary focal spot) and illuminated by a laser source within an LPP EUV source plasma chamber for plasma initiation and the generation of EUV light. It is necessary for the laser beam, such as from a high power CO2 laser source, to be focused on a position through which the target will pass and timed so as to intersect the target material when it passes through that position in order to hit the target properly to obtain a good plasma, and thus, good EUV light.
Infrared metrology is used with the EUV source to view the process of generating EUV light, for example, viewing and measuring the light reflected from the target as the target is illuminated by the laser source. Such measurements are referred to as Return Beam Diagnostics (RBD). These return beam diagnostics may include measurements of target position and shape, effectiveness of laser source illumination, laser source focus, and the like.
This metrology may be applied to a high-power laser source, or to a second laser source such as a pre-pulse laser source which illuminates the target prior to the target being illuminated by the high-power laser source.
The focus of the laser source on the target is critical to EUV production. This focus on the target changes during the operation of the EUV source, such as through thermal heating of the optics used to focus the laser source on the target, thermal expansion of other components in the EUV source affecting target focus, and the like.
What is needed, therefore, is a way to correct the focus of the laser source the target in a LPP EUV system.